The proposed investigations represent a continuation and extension of previous studies on the pathophysiology and cytokinetics of a virally induced erythroleukemia of BALB/c mice which we have designated as RLV-A disease. In particular, it is planned to: 1) Examine the role of erythroid-committed stem cells (CFU-E, BFU-E) in the pathogenesis of the erythroid dysplasia which occurs; 2) evaluate changes in RLV-A-infected myeloid stem cell (CFU-C) behavior which may be induced by exogenous agents (such as endotoxin and colony-stimulating factor) and determine if these alterations can be associated with the myeloid leukemia which sometimes terminates RLV-A disease; 3) determine the cause of the reduced red blood cell life span found in RLV-A erythroleukemia; 4) investigate (using diffusion chamber cultures) the extent to which RLV-A infected erythroid precursors may be transformed; 5) try to elucidate the role of the RLV-A virion in this erythroid dyscrasia, with special attention being directed to the relationship of RLV-A to SFFV; 6) attempt, in vitro, to produce hybrid RLV-A-SFFV bearing virions to ascertain if classical RLV-disease (MuLVR) (original source of RLV-A) can be reestablished when such hybrids are used to infect BALB/c mice; 7) study the kinetics of production and release of granulocytes to determine if subtle differences in cytokinetic behavior are induced by RLV-A infection.